1. Field of the Invention
The invention relates to the extraction of leachable mercury from insoluble mercury containing waste products. The invention more particularly relates to the removal of leachable mercury from waste sludges derived from chlor-alkali plants to render it suitable for land fill or other purposes.
2. Related Art
Ecologically and economically it is important to prevent loss of mercury from industrial processes. Operation of mercury cathode alkali-chlorine electrolysis cells are a significant source of industrial mercury consumption. Mercury solubilized in the depleted cell brine is normally returned to the electrolysis cells. Invariably, however, some mercury escapes from the process cycle when insoluble waste products designated "sludge" are purged from the electrolysis operation together with brine saturated with dissolved elemental mercury and mercury chloride complex.
Mercury cathode electrolysis brine "sludge" is a brine insoluble waste product, originating from a variety of sources in mercury cathode brine electrolysis cell operations. Clarifier sludge and saturator sludge comprise the most important types of sludges. Clarifier sludge is a precipitate formed when alkali-metal carbonate or hydroxide (e.g., sodium carbonate and/or sodium hydroxide) is added to brine to precipitate unwanted alkali earth metal and other metal ions. Saturator sludge is a residue resulting from the reconcentration of depleted brine by dissolving solid alkali-metal chlorides (e.g., sodium chloride). Sludge may also arise from other process related apparatus or operations such as cell box washing, filter backwashes, or solids contained in purge streams. Sludge of different types may be combined. Optionally, sludges may be concentrated in settling ponds to increase solids or mercury contents.
In the operation of the chlor-alkali plant, sediments from the saturator and the clarifier have traditionally been stored in ponds on the plant site. Removal of the process sludge to land fill sites is complicated by the fact that the sludge is judged hazardous if it contains over 20 ppb of mercury by Extraction Procedure Analysis as specified by E. P. A. in the Federal Reg. Vol. 43 No. 243 pg. 58952.
In the treatment of mercury containing waste materials, one widely used method is to heat the mercury containing waste in a retort to recover the metallic mercury. Caustic sludges, however, such as those obtained from an amalgam decomposer cannot be fed to a retort without causing corrosive damage to the equipment. U.S. Pat. No. 4,149,879 discloses a process for recovering mercury and caustic values from caustic (brine) sludges in a process which comprises intermixing the caustic sludge with a liquid such as water or a dilute caustic solution under pressure or agitation to produce a caustic slurry which is separated into a caustic liquid, metallic mercury and remaining sludge which is suitable for roasting in a retort to recover better than 99% of the mercury in the sludge.
It is an object of the invention to provide a process for treating mercury-containing brine sludge which substantially eliminates the pollution of natural resources.
It is an additional object of the invention to provide a process of treating mercury-containing brine sludge to achieve a leachable mercury concentration of less than 20 ppb.